The present invention relates to a polarization diversity light receiving system and, more particularly, to a polarization diversity light receiving system utilizing baseband combining which stabilizes intermediate frequencies.
In heterodyne/coherent fiber optic communication it is necessary that the planes of polarization of signal light and local oscillator light be brought into agreement with each other on a photodetector of the receiver. However, the sensitivity for receiving light varies with time because the state of polarization of the signal light undergoes variations owing to various disturbances in the optical fiber transmission line. To avoid this, the polarization diversity light receiving system is employed as one of means for implementing stable light receiving sensitivity independent of variations in the state of polarization of the signal light.
With this system, the signal light having experienced variations in its state of polarization is split into two orthogonally polarized waves at the receiving end and the two polarized waves are each detected by local oscillator light having adjusted its plane of polarization to that of the polarized wave. By electrically combining together the two received signals it is possible to suppress the influence of variations in the state of polarization.
The polarization diversity light receiving system is divided into two types in terms of the above-mentioned electrical combining method. The first is a system which electrically combines two received signals together under an intermediate-frequency condition (which system will hereinafter be referred to as "intermediate frequency combining").
The second system is one that combines the two received signals after demodulating them independently of each other (which system will hereinafter be referred to as "baseband combining").
With the intermediate frequency combining, since a phase difference between the two orthogonally polarized wave components caused by variations in the state of polarization of an signal light varies every moment, the automatic phase adjustment adder is needed for adjusting the phases of the two received signals at all times; so that this inevitably involves a complex arrangement. In contrast thereto, the baseband combining combines the received signals after demodulating them, and hence has the advantage of dispensing with the above-mentioned phase adjustment; and some practical embodiments have been proposed so far. On the other hand, the semiconductor laser for emitting the signal light or local oscillator light is defective in that the oscillation wavelength is liable to vary under the influences of external temperature changes and aging. When the oscillation wavelength (i.e. frequency) of the signal light or local oscillation light varies, no stable detection can be achieved. To solve this problem, it has already been proposed, as an intermediate frequency stabilizing method in the coherent type light receiving system, to feed back to the local oscillation laser an electrical signal obtained by discriminating the intermediate frequency (T. Okoshi, "Feasibility Study of Frequency-Division Multiplexing Optical Communication Systems Using Optical Heterodyne or Homodyne Schemes", Institute of Electronics and Communication Engineers of Japan, Paper of Technical Group, OQE78-139, 1979).
However, since the conventional polarization diversity light receiving system using intermediate frequency combining has an automatic phase adjustment adder, frequency stability is affected by fluctuations in the state of polarization of the signal light. Accordingly, there is a strong demand for an intermediate frequency stabilizing system useful for the polarization diversity light receiving system employing the baseband combining which does not involve the automatic phase adjustment adder, but no proposals have been made up to now.